Bed material, consumed absorbent and ashes formed by the fuel must be continuously fed out from a bed vessel with combustion in a fluidized bed for the bed level to be maintained at the desired level. In a power plant with combustion at a pressure exceeding the atmospheric pressure, there must be a gas-sealing sluice system in the discharge system, for a rotary sluice valve or a lock hopper system with series-connected valves and containers. The stress on the components included is great because the bed material has a high temperature and is greatly abrasive. To reduce the temperature stresses, it is known to provide, in an ash chamber below the fluidization nozzles of the combustion chamber, cooling tubes for cooling the bed material before it reaches the discharge device. In PFBC power plants with a bed vessel enclosed within a pressure vessel, it is known to cool bed material in the ash chamber by combustion air introduced into the ash chamber directly from the surrounding space in the pressure vessel. The cooling air is utilized for the combustion and the cooling device is simple. However, the possibility of cooling is limited to about 300.degree. C. because the compresssed combustion air between the pressure vessel has a high temperature. During compression, the temperature rises and in the pressure vessel it is further heated by the bed vessel and the cleaning plant in the pressure vessel.